Accident Prevention The Use of Hoses & Hard-Arms at Marine Terminals Handling Liquefied Gas Information Paper No. 4 Published in 1996 by © Society of International Gas Tanker and Terminal Operators Ltd Printed and Distributed by Witherby Seamanship International Ltd. 4 Dunlop Square, Deans Estate Livingoton, EH54 8SB United Kingdom Tel No: +44 (0)1506 463 227 Fax No: +44 (0)1506 468 999 www.witherbyseamanship.com ISBN 13: 978 1 85609 114 5 ISBN 10: 1 85609 114 7 2nd Edition - August 1996 Notice of Terms of Use While the information given in this document has been developed using the best advice available at the time of publication, it is intended purely as guidance to be used at the owner’s risk. No responsibility is accepted by the Society of International Gas Tanker and Terminal Operators Ltd, or by any person, firm, corporation or organisation who, or which, has been in any way concerned with the furnishing of information or data, the compilation, publication or authorised translation, for the accuracy of any information or advice given herein or for any omission herefrom or for any consequences whatsoever resulting directly or indirectly from the compliance with or adoption of guidance contained herein even if caused by a failure to exercise reasonable care. Notice of Terms of Use While the information given in this document has been developed using the best advice available at the time of publication, it is intended purely as guidance to be used at the owner’s risk. No responsibility is accepted by the Society of International Gas Tanker and Terminal Operators Ltd, or by any person, firm, corporation or organisation who, or which, has been in any way concerned with the furnishing of information or data, the compilation, publication or authorised translation, for the accuracy of any information or advice given herein or for any omission herefrom or for any consequences whatsoever resulting directly or indirectly from the compliance with or adoption of guidance contained herein even if caused by a failure to exercise reasonable care. WSIL - 184 Accident Prevention.indd iWSIL - 184 Accident Prevention.indd i1/10/2009 6:40:49 PM1/10/2009 6:40:49 PM Contents Page No. 1 Introduction 1 Preamble 11.1 Scope 21.2 2 Accident Summary 3 Findings 32.1 Suggested procedures 32.2 3 The Hazards 5 4 Safe Berths, Mooring Systems & Operating Procedures 7 Ship manoeuvres in harbour areas 74.1 Moorings and ship size limits 74.2 Operations and operational procedures 94.3 5 Cargo Hoses 11 Hose specification 115.1 Hose handling and operation 145.2 Hose inspection, testing and maintenance 145.3 Ship’s hoses 155.4 Ship-to-Ship transfer operations 155.5 6 Hard-Arms (Loading Arms) 17 Hydraulic analysis and surge pressures 176.1 Hard-arm specification 186.2 Hard-arm handling and operation 196.3 Hard-arm maintenance 216.4 7 Emergency Shut-Down/Release 23 Small terminals 237.1 Larger terminals 267.2 Refrigerated terminals 277.3 8 Pipelines 29 Surge pressures 298.1 Pipeline design and maintenance 338.2 9 Analysis of Incidents 35 The data-base of incidents 359.1 Frequency of accidents or near-miss 379.2 10 References 43 Appendices 45 A-1. Known accidents and near-miss data 45 A-2. Terminology and descriptive definitions 51 A-3. Recommendations for terminals-Ship break-out 53 Index 55 WSIL - 184 Accident Prevention.indd iiiWSIL - 184 Accident Prevention.indd iii1/10/2009 6:40:50 PM1/10/2009 6:40:50 PM 29 SIGTTO Pipelines 8 PIPELINES 8.1 Surge pressures In this section the question of ship loading is addressed firstly because these operations are the ones most likely to attract the highest surge pressures. Particular issues relating to cargo discharge are covered later. 8.1.1 Ship loading The problems of surge pressure on ESD can be reduced if the terminal and the ship ESD systems are linked (to act in combination) such that: (1) pumps are stopped (2a) on loading, the terminal ESD valve will shut first, and (2b) on discharging, the ship’s ESD valve will shut first. Also, it should be noted that ship’s pipelines, in the refrigerated trades, may only be designed for a 10 bar working pressure whereas those on shore will normally be designed for 19 bar. The lower pressure rating on these ships can add an extra risk to the surge problem. The foregoing points provide the main reason for including this section in the report. Additionally, there are accident reports on record pointing to surge pressure considerations. At first glance the benefit of ESD valves seems obvious. However, valve closure against cargo flow can have serious side-effects such as the bursting of pipelines, hoses or hard-arms, therefore the phenomenon of surge pressure has to be taken into account. Accordingly, when establishing the proper operation of ESD valves it is first necessary for terminals to consider how surge pressures may be generated in a pipeline system and then to establish how these effects can be contained or minimised. This is necessary not only for the shore system but also within the ship’s manifold piping. It is only after this has been done that the combination of pumping rates and valve closure times can be optimised. On this subject it is worthy of note that surge pressures can be more severe when pumping liquefied gas cargoes as compared to other products. This is because gas cargoes are only ever in association with their own gas. Products such as oil cargoes may mix with air or inert gas and, even in very small admixed quantities, this effect can produce significant buffering. Most of the arguments in this section are solved where a linked ship/shore ESD system is fitted. With this equipment, if loading, the shore valve would always be programmed to shut first. In this way the ship/shore connection is protected. Without such linkage, safe procedures can seldom be guaranteed. Reference to Section 7.2 shows that the failure of some shipboard equipment will automatically set a ship’s ESD in motion. Where no ship/shore link is fitted, the ship’s ESD valve would shut first and the connection is endangered. There have been a number of accidents of this type*. * See Appendix 1: Report 6.4 WSIL - 184 Accident Prevention.indd 29WSIL - 184 Accident Prevention.indd 291/10/2009 6:40:57 PM1/10/2009 6:40:57 PM 30 Pipelines SIGTTO Surge pressure – General In general, cargo loading rates should be set as a function of: • an hydraulic analysis of the pipeline and transfer system • the ESD valve closure time • the characteristics of the valve itself • the pipeline length and diameter from shore pump to ship, and • the availability of surge pressure relieving devices These matters are fully discussed in a reference[14] and quantified in another[18]. Once a pipeline system is in operation, when considering surge pressures, of all the matters mentioned above, a reduction of the cargo loading rate is most likely to yield immediate benefit. Accordingly, cargo pumping flow rates (in association with shore ESD valve closing times) should be set to limit surge pressure generation to below acceptable levels. Surge pressures result from rapid changes to cargo flow and reverberate, at the speed of sound (fluid dependant) through the pipeline system. They will be more severe on longer systems. In other words, if a ship is loading and only the ship’s ESD valve is closed, a worst case scenario develops. Similarly, if a ship is discharging and a valve is closed deep inside the terminal a similar risk develops*. In an emergency, given that the valve most likely to be closed is situated at the manifold area, on ESD it follows that for any particular terminal, surge pressures will be greater during cargo loading in comparison to cargo discharging. Accordingly, at loading terminals, it is often found that a quick acting bursting disc (or a quick acting pressure relief valve) is fitted in a pipeline-branch with a connection to a surge drum. The bursting disc is designed to relieve short duration surge pressures (of a damaging nature) so allowing the scantlings of terminal pipework to be optimised. To have immediate effect, the bursting disc and surge drum should be fitted close to the shore ESD valve. Where fitted, such systems have the benefit of allowing higher loading rates. Study of a typical (unprotected) ship/shore pipeline system[14] has shown that excessive pipeline pressure surges can be generated if a valve were to fully close in 15 seconds. The study also found that pipeline pressure surges are much reduced if pumps are stopped at least one pipeline-period before the valve commences closure. Pipeline-periods depend mainly on product type and line length; for example a four kilometre pipeline in refrigerated ammonia has four seconds period; but for a similar line in propane at ambient temperature the pipeline period will be about 15 seconds. Operational procedures Shore management should be guided regarding suitable valve closure times by having access to the hydraulic analysis of the cargo piping system. Generally, for any ship/terminal cargo system the optimal pumping rates will depend on the results of hydraulic analysis set against the valve closure time and pressure rating for the system. Here it will be found that slow pumping along with short pipelines can tolerate fast valve closure times but on longer lines slower valve closing times are needed. The maximum valve closure time, as allowed for ships in the IGC Code is 30 seconds. On loading it is therefore clear that shore valves must always fully close in less than 30 seconds. Ship and shore ESD valve-closure- times are adjustable to levels below 30 seconds but it should be kept in mind that valve characteristics will mean that for a 30 second total valve closing time the effective valve closing time may be as little as 1.5 seconds. This depends on valve characteristics – see references[14], [18]. * See Appendix 1: Report 3.1 WSIL - 184 Accident Prevention.indd 30WSIL - 184 Accident Prevention.indd 301/10/2009 6:40:57 PM1/10/2009 6:40:57 PM 31 SIGTTO Pipelines For loading terminals without detailed background information, and lacking special devices in pipelines to control surges, some rudimentary information is available which can help to determine suitable cargo loading rates. From an operator’s viewpoint this may be vital information in agreeing the terms of the Ship/Shore Safety Check List. Amongst other questions, the Ship/Shore Safety Check List asks: • Have maximum working pressures been agreed between ship and shore? - (Q.6) • Are emergency shut-down systems working properly? - (Q.10) • Do ship and shore know the rate of each others ESD valves? - (Q.11) On question 11 (Q. 11 above) the guidelines for the Ship/Shore Safety Check List say: “...the cargo handling rate should be so adjusted that a pressure surge evolving from automatic closure of the ESD valves does not exceed the safe working pressure of the ship or shore pipeline system. Alternatively, means may be fitted to relieve the pressure surge created, such as recirculation systems or surge drums. A written agreement should be made between the ship and shore supervisor indicating whether the cargo handling rate will be adjusted or alternative systems used; the safe cargo handling rate should be noted in this agreement”. How do supervisors address this issue in practice? How do they establish the maximum flow rate? Unless more detailed information is available to address this need operators can use the method shown overleaf. Here knowledge is needed of the terminal pipeline length, its diameter and the pump discharge pressure. Terminal personnel then check with the ship on the total valve closing time (for the ESD valve) and the type of valve (either ball/butterfly or gate). And, finally, from the graph (Fig. 10) an estimate of the maximum safe pumping rate is obtained. This graph is only pertinent for single pipelines and single hose connections. The use of the graph should be limited to refrigerated LPG. For further detail on this aspect and on more complex systems, such as those using a number of hard-arms, a reference is available[18]. WSIL - 184 Accident Prevention.indd 31WSIL - 184 Accident Prevention.indd 311/10/2009 6:40:57 PM1/10/2009 6:40:57 PM 32 Pipelines SIGTTO Fig. 10 Safe Pipeline Velocities to Limit Pressure Surge Where T = total valve closing time in seconds L = total length of pipeline in km The graph* is used in the following way. Firstly it will be noted that the horizontal axis is labelled T/L. This is the total valve closing time in seconds divided by the pipeline length in kilometres. For the example shown below, T/L equals 30. From here a vertical line is drawn to cut the appropriate curve (for pressure and valve type). At this point a horizontal line is taken to read off the maximum allowable flow rate. Reference must then be made to standard documentation to convert the pipeline velocity (m/sec) into a flow rate (m3/h). It is not safe nor is it practical to specify a universal minimum valve closure times for ESD valves. This is because terminals vary in design and size and accordingly the optimum closure time will depend on the actual system characteristics. It should be kept in mind however that ship’s valves are specified in the IGC Code as having a maximum closing time of 30 seconds and that this is the total time from first actuation to complete valve closure. * Reproduced from Reference [18] The numbers indicate the max total discharge pressure (bar g). Gate valves are indicated by the thin lines. Ball, butterfly and sluice valves are indicated by the thick lines. 8 5 5 15 15 10 10 7 6 5 4 3 2 1 0020406080100120140160180200 T/L (sec/km) Max. pipeline velocity (m/sec) Example: FLOW-RATE CALCULATION (Single Line/Single Valve) Shore ESD valve closing time Shore Valve Type 30 ..........(sees) ...................Ball Ship ESD valve closing time Ship Valve Type 30 ..........(sees) ...................Ball Pipeline Length1.0 ...........(km) Shore pumping pressure5 ..............(bar) Calculated loading flow velocity5.5........(m/sec) Line Ø = ..10?..therefore flow =1,000 m3/h Hose/Arm checked as suitable for pressure surge generation .......YES/NO WSIL - 184 Accident Prevention.indd 32WSIL - 184 Accident Prevention.indd 321/10/2009 6:40:57 PM1/10/2009 6:40:57 PM